The present invention relates to methods for effecting a sleep mode of a cell supervision circuit of an electrochemical cell which circuit is not in the sleep mode after at least one sleep command sent to the cell supervision circuit. The invention also relates to a lithium-ion rechargeable battery.
A rechargeable battery is typically used as energy source in electrically driven vehicles. Lithium-ion rechargeable batteries are often used since these have the largest energy density available to date with the lowest weight. However, said rechargeable batteries are very sensitive with respect to overcharging and also exhaustive discharge. In the case of overcharging above a particular voltage value per cell, usually about 4.25 V, the cell becomes unstable and a self-amplifying heating process (thermal runaway) can be initiated. Discharging below a particular threshold also encourages adverse chemical processes in the cell which lead to irreversible damage of said cell. For this reason, there is a rechargeable battery management system which, by means of a control device of the rechargeable battery and by means of cell supervision circuits on individual cells of the rechargeable battery, often on all of the cells, monitors the cell voltages and can be involved in triggering countermeasures. By way of example, over- and undercharging are suppressed by opening a main switching device which may comprise, for example, one or more charging or main load contactors. This is referred to as exhaustive discharge protection. During operation of an electrically driven vehicle, a situation may occur in which the installed rechargeable battery is exhausted by the driving operation, for example as a result of relatively long diversions, increased energy consumption, faulty operation of a controlled charging station or the like. Said exhaustion of the rechargeable battery is expressed in that a low voltage threshold of one or more cells is reached, which can be detected by means of the cell supervision circuits connected to the cells and upon which the exhaustive discharge protection can be activated. It is disadvantageous in this case that, even if the exhaustive discharge protection engages, that is to say the main switching device to all external consumers is open, the power supply of the cell supervision circuits is still maintained under certain conditions, for example in the case of an accident with mechanical damage to the battery control electronics, short-circuits caused by electromigration, software errors or programming errors in the rechargeable battery management system, incompatibilities or exhausted system resources. The cell supervision circuits are not part of the external consumers, but rather they remain connected to the respective electrochemical cells, even if, for example, a main contactor is open. Usually, the cell supervision circuits are not transferred into their sleep mode by the exhaustive discharge protection but rather by means of a sleep command, which can fail. In this situation, the cell supervision circuits still consume current from the rechargeable battery. Particularly when this takes place over a relatively long period of time, there is an acute danger of exhaustive discharge, especially as the rechargeable battery may already be in a state of maximum permissible discharge anyway after a relatively long driving operation.
The same may also take place when the lithium-ion rechargeable battery is transferred into a sleep mode after the electric motor has been switched off. In this case, end consumers in the low-volt range shut down their current requirement and the control device sends a sleep command to the cell supervision circuits. If a cell supervision circuit is erroneously not transferred into the sleep mode, there is a threat of an exhaustive discharge, as described above.
It is conceivable to send a sleep command to the cell supervision circuits, for example, via a CAN bus. In this case, the sending of the sleep command can be connected to a confirmation request at a cell supervision circuit and the receipt of a confirmation of the sleep mode by the cell supervision circuit, for example via a CAN bus, can be expected. If this fails, the sleep command can thus be sent again and this can be repeated for all cell supervision circuits which have not responded until all cell supervision circuits have responded. The above-mentioned method can be performed while a main switching device of the lithium-ion rechargeable battery, by means of which electrical current flow from and to the lithium-ion rechargeable battery can be interrupted, is open. Preferably, when the main switching device is opened, a signal is sent to the user about this event, particularly when the main switching device has been opened because a low state of charge of the lithium-ion rechargeable battery is present, which is preferably notified to the user. The number of sleep commands and/or confirmation requests to the cell supervision circuit can be stored, in particular in the control device which sends the sleep command and/or a confirmation request to the cell supervision circuits. Optionally, a cell supervision circuit from which no confirmation has been received at the control device after a predefined number of sleep commands and/or confirmation requests can be compulsorily isolated from its cell to be monitored such that said cell to be monitored is no longer discharged by the cell supervision circuit, in particular by means of interruption devices provided for this purpose for the connection between the cell supervision circuit and its cell and electrical connections of a compulsory disconnection device, which is preferably the control device, to said interruption devices. Preferably, the user of the lithium-ion rechargeable battery or, alternatively or additionally, a receiver who is not the user, for example a workshop, an emergency service, the manufacturer of a vehicle in which the lithium-ion rechargeable battery is installed, the manufacturer of the lithium-ion rechargeable battery or the like, is informed about such a compulsory disconnection, for example by means of a mobile telephone network.
It is disadvantageous in the method depicted above that, in the event of a system disturbance in a cell supervision circuit, for example as a result of a short-circuit caused by electromigration in the monitored cell, a software error, incompatibility or exhaustion of system resources or the like, the power supply is not transferred into the sleep mode, but rather the cell is compulsorily disconnected. This is disadvantageous because this represents an abnormal operating state the management of which causes increased expenditure.